1. Field of the Invention
This invention relates to a pressure detector, more particularly to a tire pressure detector which is installed on an inflating valve of a tire for signaling a driver when the tire is deflated to a pressure below a predetermined proper pressure.
2. Description of the Related Art
The improvement of this invention is directed to a conventional tire pressure detector which has been disclosed in U.S. Pat. No. 4,814,745.
The conventional tire pressure detector is shown in FIGS. 1 and 2 to include a plastic hollow adaptor 11 which has a lower end formed with an internally threaded portion 111 for engaging an inflating valve of a tire, and an upper end engaging threadably and tightly an open lower portion of a cap-like casing 2. The lower end of the adaptor 11 is provided with a central finger piece for actuating the inflating valve. An electric alarming unit includes a sound producing circuit 12 and batteries 13 which are mounted within the casing 2 and which are connected electrically to each other. A resilient disc member 14 has a peripheral flange clamped by a hollow screw member 16 against an inner wall of the adaptor 11. The screw member 16 is made of a conductive metal and engages an internal thread of the adaptor 11 so as to position firmly the disc member 14 within the adaptor 11. The disc member 14 has a projection 141 projecting upwardly therefrom. A conductive coil spring 17 is provided to contact the batteries 13. The coil spring 17 is then connected to a conductive plate 15 to urge the conductive plate 15 to contact the screw member 16. The screw member 16 is then connected electrically to the sound producing circuit 12 by means of a wire 161.
When the adaptor 11 is mounted on the inflating valve of an inflated tire, air from the tire can push the projection 141 of the disc member 14 upwardly to move the conductive plate 15 away from the screw member 16, as shown in FIG. 2, thereby disconnecting electrically the sound producing circuit 12 from the batteries 13. Oppositely, when the tire is deflated to a predetermined reduced pressure, biasing force of the conductive spring 17 can bias the conductive plate 15 to contact the screw member 16, as shown in FIG. 1, thereby connecting electrically the sound producing circuit 12 to the batteries 13. In this way, the sound producing circuit 12 can produce an alarming sound to signal a driver.
However, the conventional tire pressure detector still has several drawbacks:
1. In order to control precisely electrical contact between the conductive plate 15 and the screw member 16 when the tire is deflated to a predetermined reduced pressure, the conductive spring 17 has to be designed to have a desired biasing force to overcome the predetermined reduced pressure such that the conductive spring 17 has to be manufactured with relatively high precision. However, it is quite difficult to manufacture the conductive springs 17 which have identical proterties.
2. Without the use of a positioning mechanism, the conductive plate 15 easily moves away from its original position when a car installed with the conventional tire pressure detector travels at a relatively high speed or runs on a rough road. Thus, the conductive plate 15 can not contact electrically and precisely the screw member 16 at a required proper condition.
3. Because it is made of a plastic material, the adaptor 11 easily breaks down upon impact with another object, such as a pebble, when the car travels at a relatively high speed. In addition, the internally threaded portion 111 of the plastic adaptor 11 wears easily after long term use.